1. Field of the Invention
This invention pertains generally to a device for shifting microwave signals in the direct current (DC) to 20 GHz range to an arbitrary intermediate frequency between DC and 20 GHz without interference between the original and converted signals, and more specifically to a device that prevents the image-sideband associated with the intended frequency side-band to be converted to an intermediate frequency band between DC and 20 GHz.
2. Description of the Related Art
There are generally two types of conventional microwave components that may provide significant frequency translation with image rejection; the digital phase modulator and the electronic image rejection mixer. In the case of the digital phase modulator, a Serrodyne phase modulated waveform is applied to the desired electrical signal to be shifted. SEE, Cumming, THE SERRODYNE FREQUENCY TRANSLATION, Proc. of the IRE, Vol. 45, pg. 175, 1957. The achievable image rejection is limited by the number of discrete electrical signals that can be implemented in hardware, usually less than ten. This limits the achievable rejection to below 25 dB. In the case of an electronic image rejection mixer, an oscillator is used to shift the original frequency. The achievable image rejection is limited by the ability to create, from the incoming broadband microwave signal, two broadband microwave signals exactly 90 degrees out of phase with exactly the same amplitude, independent of the incoming frequency. Typical devices with 3-degree phase error and a 0.25 dB amplitude imbalance are limited in their image and carrier rejection to less than approximately 30 dB.
The electrical frequency in the existing prior art can usually operate only over a narrow bandwidth due to engineering complexities. In the mixer example, it becomes very difficult to phase and amplitude match over a broad bandwidth, therefore commercial devices are usually limited to instantaneous bandwidths of less than 1-2 octaves at microwave frequencies. In the phase shifter example, the tradeoff between creating many bits for good rejection is not compatible with broadband operation because of the many paths the signal takes; therefore commercial devices are usually limited to instantaneous bandwidths of less than 12 GHz. Currently the state of the art in optical modulator bandwidth is above 75 GHz, which is significantly above commercially available electrical mixer or phase shifter products.
The object of the invention is to provide a device for up-converting broadband electrical signals, filtering-out image frequencies in the electrical domain and then down-converting the desired frequency using optical techniques.
Another object of this invention is to obtain a frequency shifted microwave signal utilizing electro-optic modulators and band-pass electronic filters.
Another objective of this invention is to provide an analog device that does not rely upon phase or amplitude matching to achieve image rejection.
These and other objects of the invention are accomplished by an image rejecting microwave photonic downconverter using a microwave sub-carrier modulation technique without concern for image frequency interference in the shifted signal, thereby allowing telecommunications systems to downconvert densely multiplexed communications channels into a low frequency band where conventional electronics can perform signal-processing functions. Further, with the image rejecting microwave photonic downconverter, incoming microwave signals can be processed without ambiguity in direction finding applications, allowing remotable, multioctave microwave signal processing for frequency and phase determination. A plurality of lasers, a first laser providing an optical carrier that is modulated by a first electro-optic modulator with a sinusoidal electrical signal generated by a first local oscillator (LO) and a second laser providing an optical carrier that is modulated by a second electro-optic modulator with a sinusoidal electrical signal generated by the second local oscillator; are transmitted independently through two polarization-maintaining (PM) optical fibers of arbitrary length to a distant point. At the distant point, the first modulated optical signal is converted to the electrical domain using a photodetector and mixed with an input from an ultra-broadband radio frequency (RF) antenna receive-array, shifting the entire RF band to a higher frequency band equal to the original RF plus the LO frequency. This upshifted frequency band is amplified and passed through a band-pass filter to attenuate frequencies outside the up-converted pass-band. The filtered signal is electro-optically mixed with the modulated optical signal from the second local oscillator using a 3rd modulator and the resulting intermediate frequency (IF)-modulated optical signal is detected using a second photodetector and transmitted to an electrical output port of the system.